Anyways, here goes.
We're in Golden Age, cool. I think Xerxes also got his golden age at the same time, with an immortal against France, which would be how he got that far ahead in tech.
As for France, I really don't think Xerxes needs a whole lot of help. I'll attack units out in the open, and pillage French resources, but I don't think we have any need to attack French cities, nor would doing so gain us anything. (Marseilles with the wines would be nice to have, but Little Bro won it fair and square.)
I pay to investigate Marseilles to check on two things: one, yes, Xerxes is in Golden Age; and two, he doesn't have Marseilles hooked up by road. I'm going to divert our workers to hooking it up for him - that means both of us can have wines!
Sulla said to think about a wonder. Yeah, let's go for one, how does Hangchow sound? Bach's, Smith's, and Copernicus are all available soon and useful.
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Apparently, we're helping Xerxes mostly by absorbing French counterstrikes for him. French knights kill three of our riders over the next few turns, while we kill a couple of them trying to retake Marseilles. Our riders reach and pillage one French horse resource, but they have another on the far side of their territory.
We get Banking. Xerxes still has Astronomy and Metallurgy, of course. If I ask him "what can I pay for Astronomy", he says Banking. If I say "make an offer for Banking", he says Astronomy, 380g, 24/turn. I think we're allowed to accept the latter, and I do. (We can't get Metallurgy for less than Banking plus 50/turn.)
Babylon's also got 48/turn to pay for Banking, and they offered less than that, so I take it. Nobody else has anything to offer.
Xerxes has Navigation already. We can research anything in 4 turns (in Golden Age); should we? Well, since we just traded Banking to X-Bro this turn, I don't think he'll be researching Democracy right now. I start that at max research; no, we won't use it, but it's guaranteed to get us Metallurgy from X, and will be worth plenty in trade to anyone else.
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X-man captures Toulouse in 490 AD. The next turn, he signs in Russia to join the alliance against France. I don't think our brother is going to stop until Joanie is gone.
There's some traffic problems developing around Shantung; X's and our units keep getting in each other's way. I assign a couple workers to build extra roads in the area to alleviate this.
In 510 AD, he captures Grenoble. According to the world map, EGYPT has also gotten into the action; they've captured Chartres which is on France's border with JAPAN. Can't tell the players without a scorecard, but France is getting carved up fast. Egypt's capture deprives France of their last horses, too.
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Also in 520 AD, we've researched Democracy. Unfortunately, we're a bit stuck: Xerxes has Navigation, Physics, and Metallurgy, but the one he offers us for Democracy is Navigation (plus 80 gold/turn.) I'd rather get one of the useful techs. So I place Physics on the table on his side, then ask, and he wants Democracy. I hope that's okay by the rules (it's still quite a generous deal anyway.)
He immediately revolts; I wasn't sure if he'd do that while still in war. This gives us a window to research another tech before he can research any. I pick Theory of Gravity, which we should then be able to trade for Metallurgy.
Nobody else in the world has anything to offer for Democracy, and most still lack even Astronomy.
Also in 520 AD, JAPAN joins the global alliance against France.
Also in 520 AD, Xerxes razes Cherbourg. Um, what do the rules say about lending our brother help in war, if we can't keep up with his rampage?

I do have three cities building riders (Canton, Beijing, and Tsingtao), but they're getting stuck in the gridlock going through Shantung.
And Egypt has conquered a second French city over in the east.
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540 AD: our workers have hooked up Marseilles for Xerxes, so he's got wines for himself and we can buy some. We pay his request of our treasury of 386 gold plus 7/turn, and drop our lux rate to 0% for an extra 75/turn income.
And this turn, Xerxes captures Paris. Wow.
550 AD: Our Golden Age will end at the end of this turn. According to the domestic advisor, we could get Theory of Gravity in 1 turn at 70% science, but that isn't right since we
lose the Golden Age commerce boost. At 70% science, the tech would take 614 beakers. 50% science is 448 beaker production; that should stay
safely above 300 when the GA ends for us to get the tech in 2 turns. (Since we have only 19 gold in the treasury, we run the risk of bankruptcy due to lowered cash production from the Golden Age ending if we leave the slider at 70%.)
Hangchow has Copernicus due in 3, which is wrong because of GA ending. Nanking is also building a Palace, wrongly listed also as finishing in 5. This wonder can be Bach's or Smith's or Newton's, whichever becomes available.
Persia came out of his anarchy this turn, but get this - went into MONARCHY. Um, a big whoops there on Xerxes' part.

Looks like we'll get to be the research leaders from now on! I didn't build any banks, since I figured we'd be pushing ahead on research, and now we definitely will.
Tblisi is running two entertainers to keep it in WLTKD until it finishes the courthouse.
For his birthday, Xerxes gets not six, but three fine Chinese laborers! Yay, not socks!
![Party [party] [party]](/images/smilies/partytime.gif)
(Keep peeling workers off maxed size-12 cities. Xinjian and Canton are doing so this turn.)
http://www.civfanatics.net/uploads3/rbd19-550ad.zip